1. Field of the Invention
The present invention relates to disk apparatuses, e.g., optical disk apparatus, magnetic disk apparatus, etc. More particularly, the present invention relates to a disk apparatus having a reproducing system improved in the function of retry for reading data when a read error occurs.
2. Description of the Related Art
A write/read control block in a typical optical disk apparatus is arranged as shown exemplarily in FIG. 1. In this example, a head 2 is connected with a servo circuit 3, write circuit 4, and a read circuit 5, which are controlled by an upper-level controller 6.
The head 2 optically picks up information recorded on a disk 1. The servo circuit 3 controls the head 2 in either a focus direction or a track direction on the basis of the signal picked up by the head 2.
The servo circuit 3 moves the head 2 in the track direction in response to a seek instruction from the upper-level controller 6.
The write circuit 4 transfers write data to the head 2 and also controls the emission power of a light source in the head 2 in response to a write instruction from the upper-level controller 6.
The read circuit 5 receives a read-gate signal (READ GATE) from the upper-level controller 6 and binarizes the analog signal picked up by the head 2 and transfers a read clock (READ CLOCK) and read data (READ DATA), synchronized with the read clock, to the upper-level controller 6.
The upper-level controller 6 controls the servo circuit 3, the write circuit 4, and the read circuit 5.
One example of the read circuit 5 is shown in FIG. 2. In this example, the read circuit 5 is provided with a differentiating circuit 7, an AGC circuit 8, a digitizing circuit 9, a variable frequency oscillator (hereinafter referred to as "VFO") 10, and a reference clock (REFERENCE CLOCK) generator 11.
For reproduction of data, processing is executed as follows. First, an analog signal (RF signal) picked up by the head 2 is differentiated in the differentiating circuit 7 and formed into a signal with a fixed amplitude in the AGC circuit 8.
The output of the AGC circuit 8 is converted into pulse-shaped binary data (hereinafter referred to as "RAW data") in the digitizing circuit 9, and it is then input to the VFO 10.
The VFO 10 is fed with the binary data (RAW data) and a reference clock (REFERENCE CLOCK) from the reference clock generator 11 and transfers to the upper-level controller 6 a read clock (READ CLOCK) and read data (READ DATA) synchronized with the read clock.
The upper-level controller 6 detects a data start identification region from the read data and read clock and fetches data from the detected region.
At this time, an error correction (e.g., maximum 80 byte/sec) is made by an error correction function, for example, being Reed-Solomon codes.
The VFO 10 is controlled by a read-gate signal (READ GATE) from the upper-level controller 6. When the read-gate signal is asserted to be valid, the VFO 10 executes a phase-lock operation in a PLL (Phase Locked Loop) provided therein. The VFO 10 makes the frequency and phase of the binary data synchronized with the reference clock (REFERENCE CLOCK) on the basis of VFO phase-lock patterns in the binary data to generate a read clock and read data synchronized therewith. When the read-gate signal(READ GATE) is negated, the synchronization is suspended.
The timing chart of the read-gate signal (READ GATE) is shown in FIG. 3. In FIG. 3, 1 shows a data format on the disk 1, 2 the read-gate signal (-READ GATE: read-gate signal of negative logic) during writing, and 3 the read-gate signal (-READ GATE) during reading.
The read-gate signals(-READ GATE) at 2 and 3 are asserted when it is necessary to read RAW data from the preformat area and user area in the data format shown at 1 in FIG. 3.
The data format, shown at 1 in FIG. 3, is a format for one sector of a disk track, which comprises a preformat area (a preset area) and a user area (an area where data is written by the user).
The preformat area is provided with three areas, each comprising "VFO+AM+ID+CRC", following a sector mark. In FIG. 3, "VFO" represents a VFO synchronizing zone, "AM" an address mark, "ID" an ID region (identification information region), and "CRC" an error check code.
To read an ID region in the preformat area, it is necessary to read the synchronizing zone of "VFO", which precedes "ID", and execute a synchronizing operation in the VFO 10.
To read data from the user area, it is necessary to read the synchronizing zone of "VFO", which precedes the data, and execute a synchronizing operation.
It should be noted that the VFO synchronizing zones are each stored with a synchronizing pattern (VFO synchronizing pattern) necessary for a synchronizing operation in the VFO 10.
Reading of data from the ID region or the user area is enabled when the read-gate signal (-READ GATE) shown at 2 or 3 in FIG. 3 is available. That is, the ID region is read by using the read-gate signal (-READ GATE) shown at 2 or 3 in FIG. 3 whether the process concerned is a data writing or reading operation.
When the read-gate signal is asserted, a VFO synchronizing pattern, which is RAW data in the VFO synchronizing zone at 1, is detected to determine an oscillating frequency of the VFO 10.
FIG. 4 shows the process of an operation taking place from the assertion of the read-gate signal to the completion of the synchronizing operation. FIG. 5 shows an example in which a phase lock cannot be effected.
In the first operation example of the VFO 10, shown in FIG. 4, 1 shows the output signal waveform of the AGC circuit 8, and 2 is an enlarged view of a part of the waveform shown at 1. Further, 3 in FIG. 4 shows binary data (RAW data), 4 the read-gate signal, and 5 the phase lock condition.
In the illustrated example, the waveform shown at 1 and 2 is normal. For example, at time T1, the read-gate signal(Read Gate) is asserted, and the VFO 10 initiates a synchronizing operation according to a VFO synchronizing pattern (period t1).
Thereafter, the synchronizing operation is completed (normally) at time T2.
In the second operation example, shown in FIG. 5, 1 through 5 show the same as those in FIG. 4. In this example, however, the output waveform of AGC, shown at 1 and 2 is disordered, so that the VFO 10 has failed to effect phase locking.
More specifically, at time T1, the read-gate signal is asserted to initiate a synchronizing operation according to a VFO synchronizing pattern (period t1). Here, it is assumed that the waveform is disordered as illustrated due to a defect, e.g., flaw, stain, etc., on the optical disk, or a noise in the circuit.
At this time, the period of the binary data (RAW data), shown at 3, is disordered as at t1 and t3, causing the frequency of PLL to go out of control.
If the frequency of PLL goes out of control as described above, the data start identification region cannot be detected, so that the system falls into a state where synchronizing operation cannot be effected, resulting in a data read error.
Further, once the PLL frequency goes out of control, clocking (timing to data bits) can not be effected unless the read-gate signal is once negated and then reasserted to execute synchronizing operation once more.
In actual practice, the preformat area on the disk 1 contains three VFO synchronizing zones, as shown in FIG. 3. Therefore, when the read clock goes out of control at the first VFO synchronizing zone, if the read-gate signal from the upper-level controller 6 is continuously used as it is for the VFO 10, read clocks which are generated from the second and third VFO zones also go out of control.
Under the above-described circumstances, there have heretofore been the following problems:
(1) When an ID region is to be read from a disk, if the disk has a flaw or dust on its surface, the VFO circuit may fail to effect the synchronizing operation. In such a case, the read clock generated in the VFO circuit goes out of control, resulting in a failure to read the ID region. PA1 (2) If the read clock from VFO goes out of control during the data writing operation and hence it becomes impossible to read an ID region on a disk, it is necessary to write data in another region on the disk by executing alternate processing.
If the read clock goes out of control during the data reading operation, the data cannot be reproduced normally.
In such a case, a data region for one sector must be wasted.